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Heat treating of AISI 1070 versus AISI 1074?
5

Heat treating of AISI 1070 versus AISI 1074?

Heat treating of AISI 1070 versus AISI 1074?

(OP)
Hi all,

I currently have two ratchet cable cutter blades.  The first was formed by hot forging of AISI 1070.  The second was formed directly from AISI 1074 plate by wire EDM.  During service, the forged parts are routinely cracking while the EDM parts are not.

I analyzed the bulk chemical composition of each sample and both matched to their respective material grades.  However, the AISI 1070 blade additionally contained a Cr content of 0.134 wt% along with Cu of 0.167 wt%, which are not part of the 1070 specification.  

My question is if the AISI 1070 sample was heat treated in standard fashion for this type of steel, would the Cr and Cu contents prevent proper heat treating?  In other words, would the heat treat schedule have to be adjusted for the inclusion of these two alloying elements?

I would appreciate any help greatly.

Thanks!

RE: Heat treating of AISI 1070 versus AISI 1074?

coreman73;
The one piece of advice I have for you to consider is to determine the root cause of cracking in the forged, 1070 blades. Get a sample pf the cracked blade(s) and have a proper metallurgical examination performed before going down the possibly wrong path of questioning heat treatment and chemical composition.  

RE: Heat treating of AISI 1070 versus AISI 1074?

(OP)
Metengr,

I am actually going to begin metallurgical examination of the cracked blade today and will be comparing it to the EDM.  I will post my results as I get them.  But just in theory, could the extra two elements discovered in the 1070 affect heat treating in any way?  

I agree though and don't want to take the wrong path either.  The chemical results were obtained first so I figured I'd try to find out if it was as least a possibility for consideration.

RE: Heat treating of AISI 1070 versus AISI 1074?

The AISI specifications allow for residual levels of Chromium of 0.20% max and Copper of 0.35% max.  You are below these levels.

Depending on the heat treating process, these residual levels could have a minor effect the heat treating results, but without knowing the specifics of the heat treating, it is pointless to try to predict if this effect would be beneficial or detrimental to cracking tendency.  As metengr states, you need to first identify the root cause of the cracking.

rp

RE: Heat treating of AISI 1070 versus AISI 1074?

coreman,

Those levels of Cr & Cu are well within typical ranges for a carbon steel grade like 1070, and it would be exceedingly rare that they would have a detrimental effect on the properties/performance of a properly forged part.  As the others recommended, a thorough metallurgical analysis will identify the root cause.

RE: Heat treating of AISI 1070 versus AISI 1074?

(OP)
Redpicker,

The heat treat process consists of two separate heat treats for each blade type (forged and EDM).  The 1st heat treat is to harden, quench and temper the overall blade to 42-46 HRC.  The second heat treat is to induction harden the cutting edge profile only and temper to achieve an HRC of 61 minimum.

The only difference between blade types is that the forged sample was normalized before machining and heat treating.

The vendor is asking to determine why the EDM sample worked and the forged sample cracked (on the cutting edge).

Another note is that the forged sample has a thickner cutting edge than the EDM.

Thanks for the input so far guys.  I'm glad I've found a place for quality help with these type issues.

RE: Heat treating of AISI 1070 versus AISI 1074?

(OP)
I've done the metallurgical analysis on these two blades with results as follows.  The specifics of heat treat are listed in the above post.

Forged blade:
1. chemical composition matches to spec
2. microhardness of cutting edge tip area shows all values above the required minimum of 61 HRC; no surface case
3. crack path is intergranular

EDM blade:
1. chemical composition matches to spec
2. microhardness of cutting edge tip area shows a decarburized layer of approximate depth 0.010"; hardness of 48 HRC at surface with values gradually increasing until leveling out at ~60 HRC at 0.010"
3. decarburized layer found around full perimeter of part including area not affected by induction hardening.

To reiterate, the forged blade failed and the EDM blade did not. It's interesting because the forged blade demonstrated the correct cutting edge hardness while the EDM blade had the lower hardness/decarburized layer.

Both blades were used to cut 954 kcmil ACSR cable with hardness of 51 HRC.

Please let me know what you guys think.  Any and all opinions are welcome.

RE: Heat treating of AISI 1070 versus AISI 1074?

What is the grain size in the induction hardened area adjacent to the crack path?

RE: Heat treating of AISI 1070 versus AISI 1074?

What was the quenching medium for the forged blade and the EDM blade? The reason I ask is that the intergranular crack morphology that was reported is not what I would have expected.

What was the appearance of the fracture surface before metallographic examination?

RE: Heat treating of AISI 1070 versus AISI 1074?

(OP)
dbooker360,
I was not able to delineate enough grains to give an accurate grain size.  In fact, I barely saw any grains within the induction hardened zone where the crack was.  I was able to see grains but they were primarly in the not induction hardened areas.

metengr,
I probably should have been more precise about the crack mode.  For some reason, I didn't have much success getting the grains but from the few I saw around the crack was that it appeared to be intergranular.  So I am definitely not 100% sure on this.  

As for the fracture surface appearance before metallographic examination, I was not able to see it.  This part only demonstrated one VERY tiny microcrack on the cutting edge surface so it was not feasible to section it in order to see inside.  I simply made one longitudinal cut through the crack and mounted that.  I realize it's missed information but unfortunately it just wasn't possible.

The etchant I used for revealing the prior austenite grain boundaries is aqueous picric acid with wetting agent.  I will make more attempts.

I will ask the vendor about the quench medium used.

RE: Heat treating of AISI 1070 versus AISI 1074?

At least you can still use picric - my company won't allow me to use it or store it!

RE: Heat treating of AISI 1070 versus AISI 1074?

(OP)
dbooker630,
Well, my company doesn't allow the picric powder either.  I have to buy the saturated aqueous picric acid solution instead.

metengr,
The same quenching media was used for both forged and EDM blades:
Oil following austenitization during normal heat treat.
Integral polymer following induction heating.

Also, there was no evidence of oxide along the inside of crack path.  The core microstructure was fairly homogeneous inside the induction heat affected zone for both blades and was tempered martensite.  

Review of the longitudinal sections indicated a banded core for the forged blade and heavily dendritic structural orientation for the EDM blade.

I'm stumped on the root cause of failure.  Could it simply be that the forged blade needs to have a thin layer of lower hardness (to increase toughness) prior to reaching the higher required hardness range of 61 HRC?  This is the case for the EDM blade, which did just fine considering it has a thinner cutting edge (0.020") versus the forged blade (0.046").

RE: Heat treating of AISI 1070 versus AISI 1074?

From what I see here, you are getting quench cracking. Quench cracking with induction hardening is sensitive to configuration, among other things. Are the induction hardening parameters the same for the forged parts and EDM parts? You may need to tweak the parameters for the forged pieces.

RE: Heat treating of AISI 1070 versus AISI 1074?

Without the benefit metallo/fractography, and just going by the observations so far, my $0.02 is a combination of four factors listed below.  Forged vs plate may not be especially relevant; both part are wrought.  (Also you're aware of the directional properties of plate.)  So eliminating this variable, its just parts 'A' and parts 'B'.

All else equal, detrimental factors in 'A' cutters include:
1. Cu is a trash element whose presence does nothing for fx toughness, and .17% may be too high.  
2. Chromium carbides, while great for hardness, can nucleate cracks.  More Cr increased that probability.
3. Normalizing may not have been necessary and may have caused grain growth = lower toughness.

Positive factors in 'B' cutters:
4. EDM process has obviously caused a decarb layer to form in the wire's path.  This made the cutting edge tougher than part 'A' (48 vs 61 HRC).  This might be a good thing in disguise.  (Soft edge flattens out again the harder cable until the interior picks up the load ...?)

Don't know enough to comment on the apparent intergranular nature.  Also you might consider NDT for cracks somewhere in the mfg process until a cause is ascertained.
 

RE: Heat treating of AISI 1070 versus AISI 1074?

Bestwrench--I would have to go with TVP here, that the Cr and Cu levels are largely irrelevant.

RE: Heat treating of AISI 1070 versus AISI 1074?

I agree with swall, both on it is likely quench cracking (based on the information provided) and that the residual levels of copper and chromium are not likely to have a direct effect on the cracking.  Most certianly, the higher carbon content of the 1074 would have a greater affect than these residual levels.  Normalizing prior to Q & T is done to refine the GS, so it is unlikely to resulted in grain growth.

Are we dealing with two different heat treat batches, one that has seen a lot of cracking and one that has not, or do the cracked forged parts come from multiple heat treat batches and the EDM parts come from multiple batches?  It could be just a matter of when the forgings were processed, they sat around in the as-quenched condition too long while the EDM parts did not.  This isn't likely if the cracks appear from multiple batches, but if all the cracked parts came from the same heat treat batch (and the EDM parts came from a different batch), that could be important.

rp


 

RE: Heat treating of AISI 1070 versus AISI 1074?

(OP)
Thanks so much for all the exerienced advice guys.  I have some new information from the vendor.

First of all, he said that the heat treat parameters were exactly the same for both the forged (1070) and EDM (1074) blades.

He had mentioned that there was also a CNC'd version of the blade made from 1074 that performed well.  He confirmed that the 1074 blades outperformed the 1070 blades.

As far as manufacturing/batch times, he said that the EDM blades were processed several months prior to the forged blades.

From this information, could the material grade used be a contributor to failure?  Maybe the residual copper and chromium levels really have contributed to the poor performance of forged blade?  Or maybe these two steels really do require different HT parameters.  Just to reiterate, the only difference in chemical composition is that the 1070 blade contains % of these two elements while the 1074 blade is virtually completely lacking.  The carbon level is nearly identical between the two steels.

I do agree with the quench cracking though since the crack site does not show any signs of damage/deformation or surface notches/abnormalities.

RE: Heat treating of AISI 1070 versus AISI 1074?

coreman73--you have to let go of that material thing! The steels conform to AISI/SAE spec limits and you have no control over residuals as long as they also conform unless you want to write your own specs and buy expensive material. I would ask the heat treater what his recommendations would be on fine tuning the induction hardening process to mitigate quench cracking. I am thinking perhaps that increasing the quench delay and/or cutting back on the power setting for the forge pieces might be the place to start. I would also make sure that he has his quench polymer monitoring under control.

RE: Heat treating of AISI 1070 versus AISI 1074?

Agreed.  Although, you can get low residuals without resorting to expensive material.  Depending on volume, you can pick and choose from what a warehouse has in inventory, for example.  It could increase cost and lead time for the raw material, but it might be that much of an increase, just more trouble.  However, it is very unlikely that these residuals are related to the problem, so you would be increasing cost and effort for no benefit.  The residual limits permitted by the AISI standards exist so that the levels of residual elements do not affect the performance of the material.  The limits would not be 0.35 max Cu and 0.020 max Cr if these levels would lead to problems.

There is something that isn't right.  If the heat treatment parameters are exactly the same, why does the forged blade exhibit no decarb while the EDM blade has approx 0.010" decarb?  

rp

rp

RE: Heat treating of AISI 1070 versus AISI 1074?

"why does the forged blade exhibit no decarb while the EDM blade has approx 0.010" decarb?"

May be because EDM causes much higher localized temperature rise than forging?

RE: Heat treating of AISI 1070 versus AISI 1074?

(OP)
Ok, I have one more question regarding the material and then I'll drop it! haha  Could there be a logical reason to explain why the 1074 blades (the EDM sample as well as some the vendor made by CNC) consistently outperform the 1070 forged blades? I've already established that heat treating is the same (or allegedly so according to the vendor).

Regarding the decarb layer on the EDM sample, it is only present on the top of the blade with continuation to the cutting edge tip.  The entire bottom surface of the blade including at the cutting edge tip shows no such feature.  Also, the decarb layer within the induction heat treated zone at the cutting edge is approxiately 10 HRC lower than the core area (~48 HRC versus 60 HRC).  Within the regular heat treated portion away from the cutting edge, the surface hardness is approximately 97 HRB.  The structure of the HRB layer appears as pure ferrite while a tempered marteniste structure is found in the decarb layer near the cutting edge.

RE: Heat treating of AISI 1070 versus AISI 1074?

(OP)
I have attached a photo illustrating the full length of the crack.  After etching, you can clearly see that the crack extends only through the induction hardened zone and dead-ends at the transtion area.  There is no sign of local damage or deformation at the entrance to this crack.  There is also no sign of internal oxide inside the crack.  The crack is very straight with no branching.

Based on this photo and the other information, is the concensus still that this is a quench crack?

The vendor confirmed that they only saw this crack AFTER the blade was used.

RE: Heat treating of AISI 1070 versus AISI 1074?

I think the biggest problem is your heat treatment spec.  61 HRC is too high for this material.  It is going to be very brittle at that hardness.  You either need to go to a tougher grade of tool steel or reduce the hardness.  58 HRC would be more appropriate.  As a general rule, a forged cutter should out perform a machined or EDM'd cutter if it is properly processed & heat treated.

RE: Heat treating of AISI 1070 versus AISI 1074?

Given the quench crack would have formed after induction heating and during ploymer quenching would not result in a noticable oxide build-up along the crack surface. Second, you should have broke open the crack surface prior to mounting for metallographic examination using liquid nitrogen to view the fracture surface versus mounting it whole.

Yes, it looks like a quench crack given the appearance and orientation. Also, just because the crack was not observed visually does not imply there was no crack. Surface NDT would need to be performed before use to confirm a crack along the edge.

RE: Heat treating of AISI 1070 versus AISI 1074?

(OP)
Thanks dgallup and metengr.

So, if it is a quench crack then the reason that it wasn't visible until after the part was used was because it simply propagated and became more prominent?  Shouldn't there be some sign of surface damage/deformation at the crack site though or not necessarily?

metengr,
Where could I learn more about the liquid nitrogen technique?  I have not heard of this before.  The lab I work in doesn't have an SEM anyway so I would only be able to perform a macroscopic review of the fracture surface.

RE: Heat treating of AISI 1070 versus AISI 1074?

coreman73;
What this method does is allow for visual/SEM examination of an embedded crack. The crack containing region is extracted, and cooled in liquid nitrogen to induce brittle fracture, similar to a notched impact specimen. What you have is an exposed fracture surface for examination with little to no collateral damage. Obviously, this method works only for ferritic materials that exhibit a DBTT.
 

RE: Heat treating of AISI 1070 versus AISI 1074?

It may be difficult to definatively determine whether these cracks are quench cracks.  Quench cracks from in the brittle martensite as a result of the stresses resulting from the decomposition of austenite during quenching.  The stresses are formed during quenching, not the cracks.  The cracks form after quenching, which can be seconds, hours, days, or even months if stresses from the transformation are not relieved and martensite remains brittle.  Tempering will both lower the hardness (and brittleness) of the martensite as well as relieve the transformaiton stresses.

[Disclamer] It is difficult to evaluate photomicorgraphs over the internet.  Artifacts from sample preparation and the inability to examine the sample at higher as well as lower magnifications can cause such evaluations to be highly unreliable [/Disclaimer]

The photomicrograph you posted seems to indicate a wide crack existing sub-surface while near the surface, the crack appears to be very tight.  This suggests that there is a variation in residual stresses within the hardened layer.  Whether or not these residual stresses caused the crack to form, or merely assisted srevice stresses that lead to crack formaiton will be difficult to determine.  The tight crack at the surface does suggest these may be difficult to see visually on new parts, however.

There are many processing variables that could lead to this. For example, if the forged parts were processed in the winter so they were very cold when the induction hardening took place but the EDM parts were much warmer when they were hardened.  Or, maybe it is only the sharper cutting edge of the EDM blades that causes the difference.  I still am confused why the EDM blade has decarb while the forged one does not.

Regardless, a tempering cycle of 1 to 2 hours at 350F to 400F might be all that is needed if they are, in fact, quench cracks.

rp

RE: Heat treating of AISI 1070 versus AISI 1074?

I vote quench crack too, for the reasons metengr elucidated.

RE: Heat treating of AISI 1070 versus AISI 1074?

(OP)
I appreciate all the help guys.  Thanks metengr for the information on the liquid nitrogen method.  I will have to find some way for someone to possibly demonstrate this to me.

dgallup or others,
I would agree that a tool steel should be a better option for achieving such a high hardness/toughness since that's what they're primarily used for.  Is there any literature or graphs that would better prove that the 1070 steels aren't the best choice for manufacturing these cutting blades?  I would like to present at least something to the vendor as another way to correct their issue.

redpicker,
Is it not simply possible that the EDM process actually created the decarb layer in the first place and that subsequent heat treating simply was not able to get rid of it?  As for the tempering recommendation, these parts have already been tempered.  Are you suggesting a double temper?

RE: Heat treating of AISI 1070 versus AISI 1074?

Something like S7 will be much tougher.  You can get some data sheets here:
http://www.simplytoolsteel.com/data-sheets.html

I would be tempted to try your forged 1070 at a more realistic hardness first.  57 to 58 HRc.

RE: Heat treating of AISI 1070 versus AISI 1074?

I guess I am just having a hard time understanding how the EDM process would result in 0.010" of decarb.  That seems excessive to have resulted from EDM processing.

If you have some unused parts from each process, why not get some NDT performed on them to see if any have pre-existing cracks?  It may answer some questions.

Without knowing more about the application, it would be difficult to know what materials will out-preform the existing material.  To make an informed decision, the cause of the cracking will need to be better understood.  

rp

RE: Heat treating of AISI 1070 versus AISI 1074?

I'm glad you brought that up, redpicker. I can't see how EDM would cause .010" either. But, I didn't want to burden Coreman73 with another issue to sort out, so I never mentioned it.

RE: Heat treating of AISI 1070 versus AISI 1074?

(OP)
Thanks dgallup.  I will recommend them at least experiment with a lower minimum required level of hardness.

redpicker and swall,
So what would be the more expected depth of decarb from the EDM process? Also, since we're talking a bit more about it, wouldn't heat treating normally remove most of this layer anyway?  I'm now beginning to wonder why it's even still there if the EDM was done prior to heat treating, which it was.    

Do you guys have any explanation how this could be possible?  I went ahead and attached a few photomicrographs illustrating this layer.  You'll see that the depth is approximately 0.006-0.007" deep.  Microhardness data reflected this depth as well but appeared to be closer to 0.010".  This layer is nearly pure ferrite.

Etching did not reveal such layer along the induction hardened portion of the blade but microhardness did show it was still there (with higher hardness of course).  In contrast, this layer appeared as tempered martensite.

I'm about to reach frustration level on this job!  I may simply send my findings and be done with it as I hate to postpone sending out the reoprt much longer. hah

Unfortunately, I do not have more of these parts to send out for NDT.  I will suggest this to the vendor though as another option.

RE: Heat treating of AISI 1070 versus AISI 1074?

The decarb shown in the photomicrograph is consistent with what would be expected on a piece of hot rolled 1074 plate that had been subsequently austenized, quenched and tempered. Not sure where this leaves us.

RE: Heat treating of AISI 1070 versus AISI 1074?

(OP)
Thanks swall.  Does this simpy leave us at the conclusion that this layer was a result of heat treating and NOT the EDM like I had originally thought?

If so then wouldn't this also suggest that the vendor was not correct and that heat treating was really not the same for both forged and EDM blades?

Any ideas/speculatoin are certainly welcome.

RE: Heat treating of AISI 1070 versus AISI 1074?

I was suggesting that the decarb was from the original plate. Can't comment about the propensity of the heat treat process to cause decarb, as you haven't described the process.

RE: Heat treating of AISI 1070 versus AISI 1074?

(OP)
swall,

Unfortunately, I don't have details of heat treating but it was clearly stated by the vendor that the process was exactly the same for both forged and EDM parts.  Keeping that constant variable in mind, it should be safe to conclude that heat treating couldn't have caused it.

I will simply have to suggest to the vendor then that this decarb likely came from the original plate, which does make sense.  

Ok final question, and I will wrap this job up.  As a theoretical question, is there a way that a pure ferrite decarb layer originating from wrought hot rolled 1074 plate could still be present even after heat treating?  If so how?  It seems like if austenitizing was done correctly this would be impossible.

RE: Heat treating of AISI 1070 versus AISI 1074?

In order for decarb to go away, you have to add carbon via an enriched furnace atmosphere. This is known as "carbon restoration". Your layer of partial decarb is acting like a cladding of 1020 steel over 1074. If you re-austenitize and quench, you end up with the 1074 and 1020 that you started with, unless you add carbon and apply the appropriate amount of time for restoration. Carbon restoration can be kind of tricky, BTW.

RE: Heat treating of AISI 1070 versus AISI 1074?

(OP)
Thank you swall and to everyone else for helping me work through this job.  I greatly appreciate it and have learned lots along the way.

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